1. Field of the Invention
The present invention relates to determining the rate of change of fluid pressure in a sealed article. In particular, the present invention relates to determining the rate of air pressure loss from an inflated pneumatic tire over a period of time which is indicative of the air permeability of the material comprising the tire.
2. Description of the Prior Art
A pneumatic tire typically includes at least one layer of elastomeric sheet material. The tire is formed in a generally torroidal configuration with the inner periphery of the tire open. The tire is mounted on a wheel rim to seal off the open inner periphery. A valve extends from the wheel rim for conducting pressurized air into the tire.
It is known that the elastomeric sheet material used in the construction of the tire is not absolutely air impermeable. Air is gradually lost from within the tire by the air permeating through the elastomeric sheet material over a relatively long period of time. As the air permeates through the tire, the air pressure within the tire decreases. This loss of air pressure may render the tire less efficient for its intended use on a vehicle. Thus, it is important to determine the rate at which air pressure is lost from within the tire.
This rate of air pressure loss from within the tire over time is representative of the air permeability rate of the tire. The air permeability rate of the tire is of particular importance since end users of the tire, such as automobile manufacturers, often have maximum air permeability rate requirements. For example, a maximum allowable air permeability rate is typically referred to as a threshhold percentage decrease in air pressure within the tire per month.
Known ways of determining the air premeability rate of a tire exist. The known ways include monitoring the air pressure within a tire and wheel assembly over an extended period of time, such as three to six months. This amount of time is necessary to acquire a relatively large number of different air pressure readings that are needed to perform a statistically significant analysis of the tire. The air permeability rate of the tire is calculated over that time period and is compared to the threshhold air permeability rate requirement to determine acceptability.
This known way typically includes merely the visual reading of a bourdon tube air pressure gauge attached to the valve extending from the wheel rim and manually recording the air pressure. A calculation is performed on a sufficient number of the recorded air pressure readings. This known way often has drawbacks, such as in an inconsistency in reading the air pressure gauge and the lack of accuracy of the air pressure gauge.
The known way also has the disadvantage of not compensating for an increase in volume or "growth" during testing of an unaged tire. An unaged tire is a tire which has not been inflated prior to testing. The resultant increase in volume of the unaged tire proportionately decreases the air pressure within the unaged tire without a loss of air from within the unaged tire.
The known way also suffers from the disadvantage of not being able to compensate for changes in environmental conditions adjacent the tire over such an extended period of time. For example, it is known that over a three to six month time interval that the temperature immediately adjacent the tire being tested may vary. If a decrease in temperature is not compensated for, the test may yield unacceptable results for a tire that is in fact acceptable. That is, pressure in the tire may decrease without a loss of air. Thus, the rate of air pressure loss from within a tire is difficult to accurately determine.